Up to now fusible elements which had to perform a high cycling duty were generally made of silver. The rising price of silver has raised the question whether any other less expensive metal than silver could be substituted for silver. Extensive experiments were carried out with fusible elements of copper and with fusible elements of aluminum. The results of these experiments were rather unsuccessful. Copper forms on account of its oxidation a brittle layer around the fusible element inconsistent with high cycling performance.
Oxidation of copper occurs over a wide range of temperatures, beginning at room temperature and forming oxides that are not only brittle but also fissured, exposing also the underlying copper layer to oxidation. For this and other reasons, the cycling ability of prior art fusible elements of copper was extremely poor. Nor have fusible elements of aluminum been able to provide a satisfactory cycling performance.
High cycling ability fusible elements in ribbon form were, therefore, made exclusively of silver.
Prior art fusible elements designed to have a high cycling performance are disclosed, e.g. in U.S. Pat. No. 3,319,029 to P. C. Jacobs, Jr. for HIGH-VOLTAGE FUSE HAVING ZIG-ZAG SHAPED FUSE LINK; 5/9/67; U.S. Pat. No. 3,394,333 to P. C. Jacobs, Jr. for ELECTRIC FUSE HAVING STRESS-REDUCING FUSE LINK MEANS; 6/23/68; U.S. Pat. No. 4,161,713 to P. C. Jacobs, Jr. for FUSIBLE ELEMENT FOR ELECTRIC FUSES HAVING A RELATIVELY HIGH VOLTAGE RATING AND A RELATIVELY HIGH CYCLING PERFORMANCE; 7/17/79; etc. All the fusible elements described in these patents were of silver.
The present invention solves the problem of providing an inexpensive fusible element having a cycling-resist ability surpassing the cycling resist ability of any prior art fusible element.
Another object of the invention is to provide fusible elements of copper that do not oxidize and have a high cycling ability.
Other objects of this invention and advantages thereof will become more apparent as this specification proceeds.